When the malingering whorls of tropical storm Cindy swept through Hampton, Ga., Maggie Kimbrell had no reason to fear a flood, even if the nearby Atlanta Motor Speedway looked like a bass fishing lake with stands.

After all, where her city in the rolling plains of central Georgia should be on the Federal Emergency Management Agency's (FEMA) flood maps, there was only a blank space - a dereliction of planning, it turns out, as runoff from a new development created a surprise flood that inundated Ms. Kimbrell's house.

"Before they give a permit for another subdivision, they need to know how the water's going to flow," she says. "Making money's a great thing, but ruining somebody's house is not so great."

Kimbrell's complaints prompted Hampton to become one of roughly 20,000 communities that will have its sorely out-of-date flood maps upgraded. These maps allow the US to insure some $800 billion worth of low-lying property.

The $1.6 billion federal mapping project spans from the craggy trout-ways of Rockland County, N.Y., to the muddy backwaters of Alabama's Tallapoosa River, including 2,740 flood-prone communities now ineligible for insurance because the low-lying areas have not been properly mapped.

The zoning requires builders to adhere to stricter codes, but critics say the expansion of flood plain zones isn't enough to change what they call an inadequate national flood mitigation policy. Putting new regions into flood zones may actually encourage construction in them, they say, because flood insurance eases the risk for lenders and speculators.

"The paradox that we're dealing with is that, in some sense, establishing good mitigation policies does make it easier to develop in a flood plain," says Walt Peacock, director of the Hazards Reduction and Recovery Center in College Station, Texas. "One of the fundamental problems is that the policy of the United States doesn't try to prevent building in those areas in the first place."

Chiefly, the maps must pass the "red-face test": To instill confidence among local residents, homes that are 40 feet up on a hillside can no longer be shown lying in a flood plain.

"We've got some communities that haven't had a restudy in 15 or 20 years with all types of development occurring with no regulations to protect those people, creating tomorrow's disasters," says Larry Larson, executive director of the Association of State Floodplain Managers in Madison, Wis.

A few weeks ago, Hillsborough County, Fla., officials rolled out a new map marking 15,000 properties as high-risk for flooding, which had previously been classified as having a medium-risk. On the flood plains west of St. Louis, developers and conservationists are battling over flood contours as they define acceptable development and try to prevent a flooding disaster. In Tallapoosa County, Ala., new maps recently unveiled will cover hundreds of square miles of previously unmapped flood plains.

The task involves upgrading the 90,000 maps in FEMA's inventory, some of them hand-sketched, to a digital, Internet-accessible format in the next four years.

But unpredictable floodwaters can sometimes thwart mapmakers' best efforts.

"The never-ending problem, especially in rapidly growing urban areas, is that the flood likelihoods are often times way off from reality," says Mr. Peacock at the hazards center.

For communities that enroll in the federal insurance program, they have to hire building inspectors and abide by stricter building codes.

Proponents of flood insurance say this gives the local government more control over where development happens as well as the standards new buildings must abide by to withstand flooding.

Developers, though, are often leading the way to build near oceans, lakes, and streams, and buyers are following behind. Investment deals are structured so that they maintain a short-term risk, and buyers take on the long-term odds.

"Flood insurance isn't the only factor driving development, if it is one at all," says Mr. Larson. "The fact is, there are people willing to put a $4 million home in an area where they can only get $250,000 worth of insurance."

However, without a stringent federal policy that addresses flood-zone construction, more accurate maps are seen as the best tools to help local communities develop regulations that protect home buyers and allow reasonable development.

"The common man doesn't always know that if you plant a house in this particular spot, come next spring, it's going to be nine feet under water," says Hampton city planner Don King. "Nothing says that government needs to protect you from making a bad decision. But we do need to keep you from putting people in a dangerous place."